The goal of any communication system is to provide reliable service to the customer or end user. However, channels in electronic data transmission systems may become degraded due to stress on the system. In satellite communication (SATCOM) systems, for instance, stress can be imposed by transmitter/receiver degradations or failure and by out-of-tolerance conditions such as antenna pointing error, oscillator frequency drift, etc. Other types of stress may also be imposed on the system by precipitation occurring in a terrestrial microwave or satellite communication link, by interference from other sources, noise and items which may cause signal fading. The ultimate effect of communication stress is degraded signal quality or even complete outage. In a full duplex communication system employing digital data modulation (PSK or FSK), the quality or reliability of the system may be expressed in terms of an error rate which is the number of erroneously digital pulses received per unit of time.
The detection and identification of various functions of a communication system are usually performed independently by a collection of automated monitors which measure various signal parameters (signal level, noise power, transmitted power, etc.) along the signal path, as well as providing indications of the health of various subsystems. The signals from the monitors are compared against nominal values and stress is considered to be detected when a sufficient amount of degradation has occurred in one or more of the monitors. Several monitors are required to perform any effective stress detection since each monitor only responds to a subset of potential stresses. However, once communication stresses have been detected, a larger number of monitors is generally required to provide sufficient identification of the type of stresses present. Often, more than 10 different monitors may be required to reasonably identify the stresses common to a given link. Logic rules are then used to combine the indications given from each monitor in order to provide an estimate of the type of stress that the system is undergoing.
The Bit Error Rate (BER) is an absolute measure of a data channel's performance and automated BER monitors are available that can be used to detect the presence of communication stress. However, the time required to observe a single error is very long since a nominal BER may be as low as 10.sup.-12 bits per second. Furthermore, monitoring the BER directly provides no warning when slight degradations are taking place. Using BER monitors, communication system controllers can only become aware of any performance degradation after it already occurred, at which point the customers would have also detected it. Monitoring the BER, nevertheless, is still important since it represents the quality of the end product of any communication system.
A technique known as Pseudo Error Rate (PER) monitoring has been developed that provides earlier indications of degradations than a BER monitor. Several types of PER monitors are described in U.S. Pat. Nos. 4,188,615 and 4,034,340. PER monitors are now almost always included in the set of monitors used for detection and identification of stresses. The PER monitors overcome the long time intervals associated with BER monitors by making use of a second, parallel, receiver channel which is considerably degraded with respect to the main channel. Error rate estimates are, as a result, performed much quicker in the degraded channel due to the much larger number of errors occurring in that channel. However, that error rate is still indicative of degradations in the main channel since it is mathematically related to the actual error rate in the main channel. Therefore, the PER monitor can be considered as having a "gain" over a BER monitor since it amplifies the actual error rate and hence is much more responsive to slight changes in signal quality. Construction of a parallel receiver channel with a higher noise level than the main channel is, however, a rather expensive proposition.
Degraded parallel receiver channels can be simulated quite easily. Consider the well-known Eye Diagram, for instance, that is formed from the matched filtered outputs of a simple binary communication system. The signals are sampled at times equal to multiples of the symbol period T with positive values indicating the reception of one symbol while a negative sample indicates reception of the other symbol. The celebrated Eye Diagram results if the matched filtered outputs are collected over several symbol periods. The greater the eye opening is at the centre of the symbol period where samples are taken, the better the quality of the channel. An undistorted eye results for a no stress, noiseless case. However, the eye becomes distorted with the addition of noise and other communication stresses and, at the same time, the BER increases accordingly. Trained experts currently monitor links manually and rely heavily on Eye Pattern monitors which display the Eye Diagram on an oscilloscope. Although this type of monitor can provide much of the required accuracy, it has been mainly limited to manual observation and interpretation.
One way of simulating a degraded receiver channel was proposed in U.S. Pat. No. 3,721,959 by Robert A. George. The PER monitor disclosed in U.S. Pat. No. 3,721,959 counts the number of times an eye trace falls within the band around a symbol detection threshold, whereas a BER monitor counts the number of times an eye trace crosses that symbol threshold at the sampling instants over a given length of time. In other words, a BER monitor would detect when the eye is completely closed at the sampling instants whereas a PER monitor, as described in U.S. Pat. No. 3,721,959, would detect a partially closed eye at the sampling instants. This results in error rate amplification for that type of PER monitor.
There are a number of limitations with conventional automatic stress monitoring systems. First, the accuracy of current detection and identification monitoring systems needs to be considerably improved for satellite communication links. Greater error rate amplification is now required than PER monitors can currently provide for modern communication systems, which systems have an ever increasing complexity and bandwidth. In addition, the cost and complexity associated with multi-monitor systems may make it difficult to justify installing these monitors in small scale communication systems, especially if stress identification is required in addition to detection.